Wood fibre based direct pressed laminated flooring usually comprises a core of a 6-12 mm fibre board, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like material.
A laminate surface generally comprise two paper sheets, a 0.1 mm thick printed decorative paper and a transparent 0.05-0.1 mm thick overlay intended to protect the decorative paper from abrasion. The transparent overlay, which is made of α-cellulose fibres, comprises small hard and transparent aluminum oxide particles, which gives the thin surface layer a high wear resistance.
The printed decorative paper and the overlay are impregnated with melamine resin and laminated to a wood fibre based core under heat and pressure. The two papers have prior to pressing a total thickness of about 0.3 mm and they are after pressing compressed to about 0.2 mm.
Other common thin surface materials are wood veneer and foils, which are glued to a core.
The most common core material is fibreboard with high density and good stability usually called HDF—High Density Fibreboard with a density exceeding 800 kg/cm2. Sometimes also MDF—Medium Density Fibreboard—with a density of 600-800 kg/cm2 is used as core. Other core materials could also be used such as particleboard, plywood, and plastic materials and similar.
A balancing layer is generally applied on the rear side of the board in order to keep the panel flat when humidity changes.
HDF is produced as follows: Roundwood such as for example pine, larch or spruce are reduced to wood chips and then broken down into individual fibres in a refiner. The fibres are thereafter mixed with binders and then subjected to high pressure and temperature to form a board.
Floor panels with such thin surface layer are very cost effective compared to for example solid wood, stones and tiles. The major disadvantage is that properties related to deep surface structures, impact resistance and sound are inferior to the real products that they aim to copy.
Laminate floorings could be produced with very advanced designs where a printed pattern is coordinated with an embossed structure of the surface. The embossing is made during lamination when the surface is pressed against a steel sheet with an embossed structure.
The depth of the embossing is limited by the paper that could be damaged when the embossing is made with sharp edges or to a depth, which exceeds a few tenths of a millimeter. The embossing is mainly obtained by a compression of the overlay and the decorative paper. Embossed surfaces similar to a rough stone surface or a hand scraped wood surface or deep grooves that could be used to make bevels in a panel are not possible to make with the present pressing technology and with a reasonable cost structure maintaining the present technical properties and design.
Several methods have been used to increase the depths of the embossed surface structure.
It is known that one or several impregnated core paper sheets could be applied under the decorative surface paper and that deeper embossing up to 0.3-0.4 mm could be reached by a compression of the surface layer and the core paper. A similar method is described in WO 2004/067874. Such methods are expensive due to the fact that the pressure and pressing time must be increased from the normal level of 300-400 N/cm2 to about 800 N/cm2 and additional expensive material such as a core paper must be included in the panel under the decorative paper. Usually even the balancing layer has to be made thicker and this will increase the cost further.
It is also known that bevels and surface embossing could be made by a compression of the surface papers and the upper part of the core under the surface layer. Such a method is described in WO 03/078761 (Valinge Innovation AB) or WO 2006/066776. This method has several disadvantages. It could only be used in panels with a soft core such as MDF. This gives a low impact resistance and it is difficult to form high quality locking systems in such soft materials. In panels with a HDF core only limited and local embossing could be reached with conventional presses used in laminate production where the surface layer is formed and attached to a core (DPL technology). A considerable higher pressing force and longer pressing time is generally required to form surfaces similar to stone and rough wood surfaces.
Sufficient impact resistance can only be reached if thin surface layers are applied on a rather thick core with a high density. This means that the core material will be expensive and generally a board quality must be used that has better properties than what is needed for the overall stability of the panel and the strength of the locking system.
Definition of Some Terms
In the following text, the visible surface of the installed floor panel is called “front side”, while the opposite side of the floor panel, facing the sub floor, is called “rear side”. The sheet-shaped material that comprises the major part of a floor panel is called “core”. When the core has a surface layer closest to the front side and preferably also a balancing layer closest to the rear side, it forms a semi-manufacture, which is called “floor board” or “floor element” in the case where the semi-manufacture, in a subsequent operation, is divided into a plurality of floor elements. When the floor elements are machined along their edges so as to obtain their final shape with the joint system, they are called “floor panels”. By “surface layer” is meant all layers applied to the core closest to the front side and covering preferably the entire front side of the floorboard. By “decorative surface layer” is meant a layer, which is mainly intended to give the floor its decorative appearance. “Wear layer” relates to a layer, which is mainly adapted to improve the durability of the front side.
By “horizontal plane” is meant a plane, which extends parallel to the outer part of the surface layer. By “horizontally” is meant parallel to the horizontal plane. By “up” is meant towards the front face and by “down” towards the rear face.